Polybutadiene and polyisoprene based thermosetting compositions and method of manufacture thereof

ABSTRACT

An electrical substrate material is presented which comprises a thermosetting matrix which includes a polybutadiene or polyisoprene resin and an unsaturated butadiene or isoprene containing polymer in an amount of 25 to 50 vol. %; a woven glass fabric in an amount of 10 to 40 vol. %; a particulate, preferably ceramic filler in an amount of from 5 to 60 vol. %; a flame retardant and a peroxide cure initiator. A preferred composition has 18% woven glass, 41% particulate filler and 30% thermosetting matrix. The foregoing component ratios and particularly the relatively high range of particulate filler is an important feature of this invention in that this filled composite material leads to a prepreg which has very little tackiness and can therefore be easily handled by operators. This low tackiness feature allows for the use of conventional automated layup processing, including foil cladding, using one or more known roll laminators. While the prepreg of this invention is tack-free enough to be handled relatively easily by hand, it is also tacky enough to be tacked to itself using a roll laminator (e.g., nip roller) at room temperature. In addition, another important feature of this invention is the low amount of glass fabric filler relative to the higher range of particulate filler which leads to improved (lower) CTE in the Z axis or thickness direction, improved electrical performance (e.g., dissipation factor), lower cost and the ability to tailor dielectric constant through appropriate selection of particulate fillers.

BACKGROUND OF THE INVENTION

This invention relates generally to a method of making thermosettingcomposites and the resulting product Which preferably compriseselectrical circuit laminate materials. More particularly, this inventionrelates to an electrical circuit laminate comprising a (1) thermosettingresin of polybutadiene or polyisoprene; (2) a second polymer whichcross-links with the polybutadiene or polyisoprene resin; (3) a wovenfibrous web impregnated with the resin; and (4) inorganic particulatefiller such as silica, titania and the like. In accordance with animportant feature of this invention, the filler loadings for both thewoven web (fabric) and particulate filler are selected such thatindividual layers are relatively tack free thereby allowing ease ofhandling for lamination without the need for B-staging. The material ofthis invention allows for relatively low lamination temperatures.

Commonly assigned U.S. Pat. No. 5,223,568 (which is fully incorporatedherein by reference) describes a thermosetting composition which isparticularly useful for making electrical substrate materials. Ingeneral, U.S. Pat. No. 5,223,568 describes a composition formed from thesteps of:

(a) providing a moldable thermosetting composition that includes 1)polybutadiene or polyisoprene resin which is a liquid at roomtemperature and which has a molecular weight less than 5,000 and 2) asolid butadiene or isoprene-containing polymer capable of cross-linkingwith the polybutadiene or polyisoprene resin;

(b) forming the composition into a shape; and

(c) curing the composition to produce the electrical substrate materialincluding subjecting the composition to a high temperature curecondition at a temperature greater than about 250° C. and less than thedecomposition temperature of the composition. This composition thuscomprises a two component system, the first component being thepolybutadiene or polyisoprene resin and the second component being thesolid butadiene or isoprene-containing polymer; all of which aresubjected to the high temperature curing cycle (e.g., greater than 250°C.).

In preferred embodiments, the solid polymer is a thermoplastic elastomerblock copolymer.

U.S. Pat. No. 5,223,568 also describes a composition with a dielectricfiller (i.e., a material having a dielectric constant greater than about1.2 at microwave frequencies) homogeneously dispersed throughout thecomposition to the extent that when the composition is cured theproperties of the cured article, e.g., dielectric constant andcoefficient of thermal expansion, do not vary more than about 5%throughout the article.

In preferred embodiments, the composition of U.S. Pat. No. 5,223,568further includes a crosslinking agent capable of co-curing (i.e.,forming covalent bonds) with the polybutadiene or polyisoprene resinthermoplastic elastomer, or both. Examples of preferred crosslinkingagents include triallylcyanurate, diallylphthlate, divinyl benzene, amultifunctional acrylate, or combinations of these agents.

When the electrical substrate material includes a dielectric filler, thevolume % of the filler (based upon the combined volume of resin,thermoplastic elastomer, crosslinking agent (if any) and filler) isbetween 5 and 80%, inclusive. Examples of preferred fillers includetitanium dioxide (rutile and anatase), barium titanate, strontiumtitanate, silica (particles and hollow spheres); corundum, wollastonite,polytetrafluoroethylene, aramide fibers (e.g., Kevlar), fiberglass, Ba₂Ti₉ O₂₀, glass spheres, quartz, boron nitride, aluminum nitride, siliconcarbide, beryllia, or magnesia. They may be used alone or incombination.

The method disclosed in U.S. Pat. No. 5,223,568 provides a wide varietyof shaped articles having favorable isotropic thermal and dielectricproperties. These properties can be tailored to match or complementthose of ceramic materials, including Gallium Arsenide, alumina, andsilica. Thus, the cured articles can replace ceramic materials in manyelectronic and microwave applications, e.g., as specialized substratesfor high speed digital and microwave circuits. Examples of microwavecircuits including microstrip circuits, microstrip antennas, andstripline circuits. The cured products are also useful as rod antennasand chip carriers.

While well suited for its intended purposes, the circuit laminatematerials described in U.S. Pat. No. 5,223,568 do suffer from severaldrawbacks. For example, these prior art materials require a hightemperature cure (e.g., lamination) of greater than about 250° C. (482°F.); and this requirement is problematic for several reasons. First,conventional circuit fabrication equipment often exhibit temperaturelimits of 360° F. which is below that of the 482° F. requirement. Stillanother drawback is that flammability ratings required by UnderwritersLaboratory UL 94-VO lead to the need for inclusion of a flame retardantadditive such as a bromine-containing fire retardant. Unfortunately,typical bromine-containing fire retardant additives cannot withstand thehigh temperature cure conditions of greater than 250° C.; and willundergo decomposition and/or chemical degradation at these temperatures.

In addition to the foregoing need for lowering the cure (lamination)temperature of the circuit laminates, there is also a problem with theinherent tackiness associated with the polybutadiene or polyisoprenelaminate prepregs. Because of the tacky nature of the prepreg, it is notpractically feasible to use a continuous, automated layup process forlaminating circuit materials. Roll lamination equipment is commonlyavailable and the ability to provide a polybutadiene or polyisoprenebased prepreg of the type described in U.S. Pat. No. 5,223,568 whichalso has the ability to be used in an automatic layup process willgreatly reduce the manufacturing cost and processing time leading to asignificant increase in the commercial success of the resultant circuitmaterial products.

In addition to U.S. Pat. No. 5,223,568, there are other prior artpatents and literature of some interest which respect to the use offilled circuit laminates in general, and polybutadiene based circuitlaminates in particular. For example, in an article entitled "A NewFlame Retardant 1,2-Polybutadiene Laminate" by N. Sawatari et al, IEEETransactions on Electrical Insulation, Vo. EI-18, No. 2, April 1983, thelimitations on presently used polybutadiene based laminates is discussedincluding the fact that 1,2-Polybutadiene (PBD) is difficult to controlin the semicured condition (B-stage), difficult to make non-tacky,highly flammable, and exhibits low copper bond. The article thereafterdescribes a composition which addresses these issues. The compositiondescribed uses a high percentage of very high molecular weightpolybutadiene (PBD) to eliminate tackiness and B-staging. Also, a lowmolecular weight, modified PBD resin is used (as a minor component) toaid in copper bond and flow during lamination. There is no mention ofusing filler of any type.

U.S. Pat. No. 5,264,065 to Kohm describes a base material for printedwiring boards where inert filler is used to control Z-axis CTE infiberglass reinforced thermoset resins. The Kohm patent discloses arange of 45-65 weight % fiberglass reinforcement and a range of 30 to100 parts filler per 100 parts of the polymer. There is no disclosure inKohm of a polybutadiene or like material for the resin system.

U.S. Pat. No. 4,997,702 to Gazit et al discloses a circuit laminatehaving an epoxy resin system which also includes inorganic fillers orfibers in the range of 20-70 weight % of the total composite. The fibersinclude both glass and polymeric fibers and the fillers include clay ormineral (e.g., silica) particulate fillers.

U.S. Pat. No. 4,241,132 to Pratt et al discloses an insulating boardcomprising a polymeric matrix such as polybutadiene and a fillerconsisting of polymeric filler (e.g., fibrous polyproylene). In allcases, the dielectric constant or dissipation factor of the resin matrixis matched to the fibrous reinforcement in order to obtain an isotropiccomposite.

European Patent No. 0 202 488 A2 discloses a polybutadiene basedlaminate wherein a high molecular weight, bromine-containing prepolymeris used to reduce tack and flammability of a 1,2-polybutadiene resin.Similarly in Japanese Patent No. 04,258,658, a high molecular weightcompound is added to a tacky PBD resin to control tack. The compoundutilized is a halogen-containing bismaleimide which providesflammability resistance as well as good copper bonding and heatresistance. There is no mention of the use of fillers and the resultinglaminate will have a relatively high dissipation factor.

An article entitled "1,2-Polybutadienes-High Performance Resins for theElectrical Industry", R. E. Drake, ANTEC '84 pp. 730-733 (1984),generally discloses conventional polybutadiene resins for use inlaminates and specifically discloses the use of reactive monomers toco-cure with the PBD.

U.K. Patent Application 2 172 892 A generally discloses laminatescomposed of styrene-containing and thermoplastic copolymers withunsaturated double bonds and polybutadiene.

Notwithstanding the foregoing examples of laminate composites, therecontinues to be a need for improved polybutadiene laminates having acombination of electrical, chemical, mechanical and thermal propertieswhich are not presently available including flame retardance, improvedCTE, ability to tailor dielectric constant, improved dissipation factor,low cost and low tackiness.

SUMMARY OF THE INVENTION

The above-discussed and other drawbacks and deficiencies of the priorart are overcome or alleviated by the electrical substrate material ofthe present invention. In accordance with the present invention, anelectrical substrate material is provided which comprises athermosetting matrix which includes a polybutadiene or polyisopreneresin and an unsaturated butadiene or isoprene containing polymer in anamount of 25 to 50 vol. %; a woven glass fabric in an amount of 10 to 40vol. %; a particulate, preferably ceramic filler in an amount of from 5to 60 vol. %; a flame retardant and a peroxide cure initiator. Apreferred composition has 18% woven glass, 41% particulate filler and30% thermosetting matrix.

The foregoing component ratios and particularly relatively high range ofparticulate filler is an important feature of this invention in thatthis filled composite material leads to a prepreg which has very littletackiness and can therefore be easily handled by operators. This lowtackiness feature allows for the use of conventional automated layupprocessing, including foil cladding, using one or more known rolllaminators. While the prepreg of this invention is tack-free enough tobe handled relatively easily by hand, it is also tacky enough to betacked to itself using a roll laminator (e.g., nip roller) at roomtemperature. In addition, another important feature of this invention isthe low amount of glass fabric filler relative to the higher range ofparticulate filler which leads to improved (lower) CTE in the Z axis orthickness direction, improved electrical performance (e.g., dissipationfactor), lower cost and the ability to tailor dielectric constantthrough appropriate selection of particulate fillers.

Still another important feature of this invention is that the curetemperature for lamination is significantly lower than required by U.S.Pat. No. 5,223,568 with lamination temperatures typically in the rangeof 330° to 425° F. The low temperature cure conditions allow for the useof bromine containing fire retardant additives as well as permitting theuse of conventional circuit fabrication (e.g., lamination) equipment.The relatively low temperature cure is achieved using an appropriateamount of organic peroxide such as dicumyl peroxide and t-butylperbenzoate peroxide.

The electrical substrate material of this invention includes a pluralityof woven webs (such as E-glass webs) embedded in a mixture of thepolybutadiene or polyisoprene based resin system and inorganic filler(e.g., silica) laminated between one or two sheets of conductive foils(e.g., copper) to produce a circuit board material which is especiallywell suited for microwave applications. Of course, if very thin (e.g.,less than 3 mil thickness) cross-sections are desired, then only asingle saturated web may be used for the dielectric.

The above-discussed and other features and advantages of the presentinvention will be appreciated and understood by those of ordinary skillin the art from the following detailed description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings, wherein like elements are numbered alikein the several FIGURES:

FIG. 1 is a schematic representation of a processing line for thecontinuous manufacture of the prepreg in accordance with the presentinvention;

FIG. 2 is a cross-sectional elevation view of a circuit substratematerial in accordance with the present invention; and

FIGS. 3-5 are test data depicting the improved (lower) tackiness of theB-staged circuit substrate in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention comprises a thermosetting matrix in the amount of25 to 50 vol. %, a glass woven fabric in the amount of 10 to 40 vol. %and at least one type of particulate filler in the amount of 5 to 60vol. %. In addition, the present invention includes one or more organicperoxide initiators in an amount of 1.5 to 6 PHR and a brominecontaining fire retardant additive in an amount effective to providefire retardance.

The resin system, fillers, cross-linking agents, woven web, fireretardant additives, peroxide initiators, processing conditions andrepresentative constructions together with Examples will now bediscussed in detail.

Resin System

The resin system used in the electrical substrate material of thisinvention is a thermosetting composition generally comprising (1) apolybutadiene or polyisoprene resin (or mixture thereof); and (2) anunsaturated butadiene or isoprene-containing polymer capable ofparticipating in cross-linking with the polybutadiene or polyisopreneresin during cure. The polybutadiene or polyisoprene resins may beliquid or solid at room temperature. Liquid resins may have a molecularweight greater than 5,000 but preferably have a molecular weight of lessthan 5,000 (most preferably between 1,000 or 3,000). The preferablyliquid (at room temperature) resin portion maintains the viscosity ofthe composition at a manageable level during processing. It alsocrosslinks during cure. Polybutadiene and polyisoprene resins having atleast 90% 1,2 addition by weight are preferred because they exhibit thegreatest crosslink density upon cure owing to the large number ofpendent vinyl groups available for crosslinking. High crosslinkdensities are desirable because the electrical circuit substratesexhibit superior high temperature properties. A preferred resin is B3000resin, a low molecular weight polybutadiene liquid resin having greaterthan 90 wt. % 1,2 addition. B3000 resin is commercially available fromNippon Soda, Ltd.

The unsaturated polymer preferably comprises a thermoplastic elastomerand more preferably includes a linear or graft-type block copolymer thatpreferably has a polybutadiene or polyisoprene block with at least 50%by weight, 1,2 addition and a thermoplastic block that preferably isstyrene or α-methyl styrene. The high proportion of 1,2 addition in thepolyisoprene or polybutadiene block leads to high crosslink densitiesafter the curing step, as is the case with the polybutadiene orpolyisoprene resin described above. A preferred copolymer is astyrene-butadiene-styrene triblock copolymer, e.g., Kraton DX1300(commercially available from Shell Chemical Corp.).

The thermoplastic elastomer may also contain a second block copolymersimilar to the first except that the polybutadiene or polyisoprene blockis hydrogenated, thereby forming a polyethylene block (in the case ofpolybutadiene) or an ethylene-propylene copolymer (in the case ofpolyisoprene). When used in conjunction with the first copolymer,materials with greater "toughness" can be produced. Where it is desiredto use this second block copolymer, a preferred material is KratonGX1855 (commercially available from Shell Chemical Corp.) which isbelieved to be a mixture of styrene-high 1,2 butadiene-styrene blockcopolymer and styrene-(ethylene-propylene)-styrene block copolymer.Particularly preferred compositions are those in which the resin ispolybutadiene, the first block copolymer is styrene-butadiene-styrenetriblock copolymer (m=n=1), and the second block isstyrene-(ethylene-propylene)-styrene triblock copolymer (m=n=1), theethylene-propylene block being the hydrogenated form of an isopreneblock.

Thus, in a preferred embodiment, the unsaturated polymer comprises asolid thermoplastic elastomer block copolymer having the formula X_(m)(Y-X)_(n) (linear copolymer) or ##STR1## (graft polymer) where Y is apolybutadiene or polyisoprene block, X is a thermoplastic block, and mand n represent the average block numbers in the copolymer, m being 0 or1 and n being at least 1. The composition may further include a secondthermoplastic elastomer block copolymer having the formula W_(p)-(Z-W)_(q) (linear copolymer) or ##STR2## (graft copolymer) where Z is apolyethylene or ethylene-propylene copolymer block, W is a thermoplasticblock, and p and q represent the average block numbers in the copolymer,p being 0 and 1 and q being at least 1.

Preferably, the polybutadiene or polyisoprene resin and thepolybutadiene or polyisoprene block of the first block copolymer makingup the thermoplastic elastomer have at least 90% by weight 1,2 addition.The volume to volume ratio of the polybutadiene or polyisoprene to thethermoplastic elastomer preferably is between 1:9 and 9:1, inclusive.

Other free radical curable polymers which can co-cure with butadienepolymers may be added (such that 1,2-butadiene polymers are still themajor polymeric ingredient) for specific property or processingmodifications. Such possible modification purposes include toughness,adherability to copper foil and copper plating, and pre-preg handlingcharacteristics. These co-curable polymers include random and blockcopolymers of primarily 1,3-addition butadiene or isoprene with styrene,alpha-methyl styrene, acrylate or methacrylate, or acrylonitrilemonomers; homopolymers or copolymers of ethylene, such as polyethylene,ethylene-propylene copolymer and ethylene-propylene-diene terpolymers,ethylene-ethylene oxide copolymers; natural rubber; norbornene polymerssuch as polydicyclopentadiene; hydrogenated diene polymers such ashydrogenated styrene-isoprene-styrene copolymers andbutadiene-acrylonitrile copolymers; and others. Levels of theseco-curable polymers should be less than 50% of the total polymericcomponent.

Particulate Filler Material

The volume % of the filler (based upon the combined volume of the resinsystem, woven fabric and particulate filler) is between 5 and 60%,inclusive and preferably between 30% and 50%. Examples of preferredfillers include titanium dioxide (rutile and anatase), barium titanate,strontium titanate, silica (particles and hollow spheres) includingfused amorphous silica; corundum, wollastonite, aramide fibers (e.g.,Kevlar), fiberglass, Ba₂ Ti₉ O₂₀, glass spheres, quartz, boron nitride,aluminum nitride, silicon carbide, beryllia, alumina or magnesia. Theymay be used alone or in combination.

In an important and preferred feature of this invention, the particulatefiller is present in an amount which is (1) greater than the amount (involume %) of thermosetting composition (preferably the ratio of fillerto thermosetting composition is 45:55) and (2) greater than the amount(in volume %) of the woven fabric.

Particularly preferred fillers are rutile titanium dioxide and amorphoussilica because these fillers have a high and low dielectric constant,respectively, thereby permitting a broad range of dielectric constantscombined with a low dissipation factor to be achieved in the final curedproduct by adjusting the respective amounts of the two fillers in thecomposition. To improve adhesion between the fillers and resin, couplingagents, e.g., silanes, are preferably used.

As will be discussed hereinafter, the material described herein ispreferably used as a dielectric substrate in a circuit laminate whereina layer of metal is laminated thereto. Preferably, the filler materialand quantity thereof is selected so as to provide the substrate with acoefficient of thermal expansion which is equal or substantially equalto the coefficient of thermal expansion of the metal layer.

Woven Web

The fiber reinforcement comprises woven, thermally stable webs of asuitable fiber, preferably glass (E, S, and D glass) or high temperaturepolyester fibers (e.g., KODEL from Eastman Kodak). The web is present inan amount of about 10 to 40 vol. %, and preferably about 15 to 25 vol. %with respect to the entire laminate. Such thermally stable fiberreinforcement provide the laminate with a means of controlling shrinkageupon cure within the plane of the laminate. In addition, the use of thewoven web reinforcement renders a dielectric substrate with a relativelyhigh mechanical strength.

Preferred examples of the woven fiberglass web used in the presentinvention are set forth in the following Table 1.

                  TABLE 1                                                         ______________________________________                                        FIBERGLASS WOVEN WEBS                                                         Manufacturer   Style   Thickness (in.)                                        ______________________________________                                        Fiber Glast    519-A   0.0015                                                 Clark-Schwebel 112     0.0032                                                 Clark-Schwebel 1080    0.0025                                                 Burlington     106     0.0015                                                 Burlington     7628    0.0068                                                 ______________________________________                                    

Fire Retardant Additive

Preferably, the present invention includes a flame retardant such as abromine containing flame retardant in an amount of 20 to 60 vol. % or 2to 6 PHR. Examples of suitable brominated flame retardants includeSaytex BT 93W (Ethylene bistetrabromophthal imide), Saytex 120(tetradecabromo diphenoxy benzene) or, Sayrex 102 (Decabromo diphenoxyloxide).

Curing Agent

A curing agent is added to the composition to accelerate the curingreaction. When the composition is heated, the curing agent decomposes toform free radicals, which then initiate crosslinking of the polymericchains. Preferred curing agents are free radical cure initiators such asorganic peroxides, e.g., dicumyl peroxide, t-butylperbenzoate andt-butylperoxy hexyne-3, all of which are commercially available. Theperoxide curing agent is provided in an amount of between 1.5 and 6(PHR).

Processing

In general, the thermosetting compositions are processed as follows.First, the (polybutadiene or polyisoprene resin, thermoplasticelastomer, particulate fillers, curing agents, flame retardants, andcoupling agent (if any) are thoroughly mixed to form a slurry inconventional mixing equipment. The mixing temperature is regulated toavoid substantial decomposition of the curing agent (and thus prematurecure). Mixing continues until the particulate filler is uniformlydispersed throughout the resin. The particulate filler may be pretreatedwith coupling agents (preferably silanes) in a separate step for moreefficient use of the agents.

Next, conventional prepreg manufacturing methods can be employed.Typically the web is impregnated with the slurry, metered to the correctthickness, and then the solvent is removed (evaporated) to form aprepreg.

One processing system useful in making a prepreg in accordance with thepresent invention is shown in FIG. 1 and includes (from left to right) aroll 10 of woven glass web 11 which unwinds through a first accumulator12 to a series of drive rolls 14. The web 11 then passes into thecoating area 16 where the web is passed through a saturation tank 20(which contains the mixture of polymer, particulate filler, solvent andother components) and then through a pair of metering rolls 22. The web11 thereafter travels the length of a drying tower 18 for a selectedperiod of time until the solvent is evaporated from the web whereuponthe web passes through drive rolls 28, a second accumulator 30 andfinally web 11 (which is now a prepreg) is wound onto roll 32.

The lamination process entails a stack-up of one or more prepreg layersbetween one or two sheets of conductive foil (copper). This stack-up isthen densifted and cured via lamination or a combination of laminationand oven baking.

The stack-up is cured in a conventional peroxide cure step; typical curetemperatures are between 330° and 425° F. (165° to 218° C.). Inaccordance with an important feature of this invention, unlike U.S. Pat.No. 5,223,568, no additional high temperature cure step is needed toincrease crosslink density. It will be appreciated that the '568 patentrequires a high temperature cure where the temperature is greater thanabout 250° C. but less than the decomposition temperature of the resin(typically about 400° C.).

Referring now to FIG. 2, a cross-sectional view of electrical substratematerial in accordance with the present invention is shown generally at40. Electrical substrate 40 has been laminated in accordance with one ofthe processes described above wherein a woven web 42 is impregnated witha resin system/filler composition 44 and laminated between two copperfoils 46, 46' to produce a circuit board laminate. As discussed abovewith reference to the processing conditions, the resin system 44 mayeither be cast onto woven web 42 using known prepreg manufacturingequipment or web 42 may be saturated by resin system 44 by sandwichingweb 42 between a pair of bond plys formed from resin system 44 andlaminating the stack up together with the copper cladding 46, 46'. WhileFIG. 2 depicts a single layer of woven web 42, it will be appreciatedthat typically a plurality of layers of saturated web 12 will be used informing circuit laminates in accordance with the present invention.However, a single layer as shown in FIG. 2 is desirable where very thincross-sections (less than 3 mils) are required.

EXAMPLES

The following non-limiting examples further describe the electricalsubstrate material of the present invention.

A. Tackiness

In an effort to quantify the tackiness of the prepreg of this invention,a peel test was developed and applied to a number of samples.

Test Description

Attempting to simulate a 90° peel test, an Instron (Model 1125) was usedto peel apart 2 layers of prepreg. The individual layers of prepreg were1" wide and 12" long and were stuck together by rolling a 10 lb rollerover them in the lengthwise direction. A 50 lb tensile load cell wasused in conjunction with a 15 ipm crosshead speed to measure the peelforce or tack. For verification of the data, the tests were duplicatedusing a 10 lb cell and 12 ipm.

Test Samples

Four prepreg formulations were created for this test. Using only 106style fiberglass, prepregs were made in which the particulate filler,resin and rubber contents varied.

    ______________________________________                                        Prepreg Component Quantities - (all % BVOL)                                                    Resin    Rubber Woven                                        Sample  Filler   (PBD)    (SBS)  Glass  Other                                 ______________________________________                                        2075-62-1                                                                             25       32       20     18     5                                     2075-62-2                                                                             34       26       16     18     6                                     2075-62-3                                                                             25       41       11     18     5                                     2075-62-5                                                                             34       34        9     18     5                                     ______________________________________                                    

Results

The measured peel values range from 0 to 0.35 pli. FIG. 3 plots the tackversus SBS (rubber) content for the two filler loadings used. From thisplot, the resulting tack is more sensitive to rubber content at thelower filler loading. Alternatively, FIG. 4 plots the tack versus fillercontent. In this case, the resulting tack is more sensitive to fillercontent at the lower rubber loading. FIGS. 3 and 4 lead to theconclusion that the combination of filler and rubber volumes effecttack. If one looks at the resulting resin contents versus tack (FIG. 5),there is a good correlation between resin volume present and tack. Putsimply, it appears that an important factor for tackiness is the amountof resin present and to reduce tack it does not matter whether the resinis displaced by filler or rubber so long as the resin is displaced.

B. Example Formulations:

The following five examples show representative electrical, thermal andmechanical data on the laminate of the present invention. In accordancewith an important feature of this invention, the dissipation factor ofthe resultant laminate is less than or equal to 0.007 which renders thismaterial well suited for use in microwave applications.

Example 1:

This is a preferred embodiment. The laminate consisted of five layers ofprepreg for a total dielectric thickness of 0.023". Formulation detailsare contained in Table 1. Note the high filler and low rubber contents.Table 2 briefly reports a few of the resulting properties.

Example 2:

This sample contained a particulate filler loading lower than thepreferred embodiment but had a higher rubber content. The laminate wasmade with five layers of 1080 woven glass resulting in a dielectricthickness of 0.021".

Example 3:

This sample contained titania (TiO₂) particulate filler compared to thesilica (SiO₂) found in the other examples. The laminate contained 10layers of 1080 woven glass resulting in a dielectric thickness of0.028".

Example 4:

This laminate had the rubber replaced in the formulation with moreresin. The laminate's construction utilized four layers of 1080 wovenglass and resulted in a dielectric thickness of 0.016".

Example 5:

This sample contained more rubber than resin. The resulting laminate was0.020" thick and utilized five layers of 1080 woven glass.

    ______________________________________                                        Examples 1-5 Constructions                                                    (units = % BWT)                                                               Example      1       2       3     4     5                                    ______________________________________                                        B3000 Resin  14.6    13.5    12    21.2  4.4                                  D1300 Rubber 3.8     9.3     3     0     13.1                                 FB-35 Silica 0       44.7    0     0     51.3                                 Minsil 5 Silica                                                                            51      0       0     45.7  0                                    Ticon HG Titania                                                                           0       0       53.2  0     0                                    Woven Glass  25      26      27    26    25                                   Silane       0.5     0.4     0.6   0.4   0.5                                  Brominated Flame                                                                           4.5     4.6     3.7   5.3   4.4                                  Retardant                                                                     Catalyst (peroxide)                                                                        0.6     1.5     0.5   1.4   1.3                                  ______________________________________                                         B3000 resin by Nippon Soda, D1300 rubber (Kraton) by Shell, FB35 silica i     fused silica from Denka, Minsil 5 silica is a fused silica from Minco,        Ticon HG titania by TAM woven glass by ClarkSchwebel.                    

    ______________________________________                                        Examples 1-5 Properties                                                       Example      1       2       3     4     5                                    ______________________________________                                        Dielectric Constant                                                                        3.47    3.35    9.54  3.22  3.42                                 Dissipation Factor                                                                         .0035   .0042   .0059 .0053 .0035                                Specific Gravity                                                                           1.83    1.75    2.47  1.76  1.83                                 (g/cc)                                                                        Copper Bond (pli)                                                                          4.4     5       2.1   3.2   3.7                                  Water Absorption (%)                                                                       N/A     0.09    N/A   N/A   0.67                                 ______________________________________                                         The DK values reported above are actually the averages of the measured        Dk's from a 1-10 Ghz frequency sweep. The Df values reported above are th     lowest recorded value of a given 1-10 Ghz frequency sweep.                    Specific gravity test method: ASTM D79291                                     Copper bond: IPCTM-650 2.4.8                                                  Water absorption IPCTM-650 2.6.2.1 (w/48 hr exp.)                        

In general, the preferred formulations of the present invention minimizefiber content and maximize particulate filler content. This highparticulate filler content (preferred ratio of resin to filler is 45:55)leads to improved (lower) CTE in the Z-axis direction, improvedelectrical performance, lower cost and other features and advantages.

While the prior art discussed above generally discloses filledpolybutadiene laminates and laminates based on other resin systems whichemploy fiber and particulate fillers, no prior art to which Applicantsare aware utilize the combination of:

1. thermosetting matrix comprised of polybutadiene or polyisoprene resinand an unsaturated butadiene or isoprene containing polymer in an amountof 25 to 50 volume %;

2. woven fabric reinforcement in an amount from 10 to 40 vol. %; and

3. particulate filler in an amount of 5 to 60 volume %.

In general, the present invention utilizes a higher particulate fillerlevel and a lower woven fabric level. Preferably, the use in thethermosetting composition of a high molecular weight copolymer permitsthe use of these higher levels of particulate fillers, all of whichleads to improved theological control, reduced tack and reducedshrinkage. More particularly, the combination of an extremely highloading level of particulate filler, woven glass and an unsaturatedpolymer while still being processable using standard industry equipmentrepresents an important advance in the circuit materials field. Inaddition, the unexpected result that the particulate filler can be usedto eliminate tackiness of the liquid polybutadiene resin representsstill another important feature of this invention.

While preferred embodiments have been shown and described, variousmodifications and substitutions may be made thereto without departingfrom the spirit and scope of the invention. Accordingly, it is to beunderstood that the present invention has been described by way ofillustrations and not limitation.

What is claimed is:
 1. An electrical circuit material consistingessentially of:(a) a thermosetting composition comprising apolybutadiene or polyisoprene resin and an unsaturated butadiene orisoprene containing polymer capable of participating in crosslinkingwith said polybutadiene or polyisoprene resin in an amount of from 25 to50 volume percent with respect to the entire electrical circuitmaterial, said polybutadiene and polyisoprene resin being chemicallydistinct from said unsaturated butadiene or isoprene containing polymer;(b) a woven fabric in an amount of from 10 to 40 volume percent withrespect to the entire electrical circuit material; and (c) a particulatefiller in an amount of from 5 to 60 volume percent with respect to theentire electrical circuit material.
 2. The material of claim 1wherein:the woven fabric is glass fabric.
 3. The material of claim 1wherein:said particulate filler is present in an amount by volume whichis greater than the amount of woven fabric.
 4. The material of claim 1wherein:said particulate filler is present in an amount by volume whichis greater than the amount of thermosetting composition.
 5. The materialof claim 3 wherein:said particulate filler is present in an amount byvolume which is greater than the amount of thermosetting composition. 6.The material of claim 1 wherein:said polybutadiene or polyisoprene resinis liquid at room temperature.
 7. The material of claim 6 wherein:saidpolybutadiene or polyisoprene resin has a molecular weight less than5,000.
 8. The material of claim 1 wherein:said polybutadiene orpolyisoprene resin has a molecular weight less than 5,000.
 9. Thematerial of claim 1 wherein:said polybutadiene or polyisoprene resin isa solid at room temperature.
 10. The material of claim 1 wherein:saidunsaturated butadiene or isoprene containing polymer comprises a solidpolymer.
 11. The material of claim 1 wherein:said unsaturated butadieneor isoprene containing polymer comprises a liquid polymer.
 12. Thematerial of claim 1 wherein said polybutadiene or polyisoprene resinincludes pendent vinyl groups available for crosslinking.
 13. Thematerial of claim 1 wherein said unsaturated butadiene or isoprenecontaining polymer has pendent vinyl groups available for crosslinking.14. The material of claim 1 wherein said unsaturated polymer is acopolymer of isoprene or butadiene and a second monomer.
 15. Thematerial of claim 14 wherein said second monomer is styrene or α-methylstyrene.
 16. The material of claim 15 wherein said copolymer is athermoplastic elastomer block copolymer having the formula X_(m)-(Y-X)_(n) or the formula ##STR3## where in each formula Y is a blockcomprising isoprene or butadiene units, X is a thermoplastic block, andm and n represent the average block numbers in said copolymer, m being 0or 1 and n being at least
 1. 17. The material of claim 1 wherein saidunsaturated butadiene or isoprene containing polymers comprises athermoplastic elastomer block copolymer having the formula W_(p)-(Z-W)_(q) or the formula ##STR4## where in each formula Z is apolyethylene or ethylene-propylene copolymer block, W is a thermoplasticblock, and p and q represent the average block numbers in saidcopolymer, p being 0 or 1 and q being at least
 1. 18. The material ofclaim 13 wherein said unsaturated polymer is a styrene-butadiene-styrenetriblock copolymer.
 19. The material of claim 13 wherein saidunsaturated polymer is a styrene-butadiene-styrene triblock copolymer,and said block copolymer is a styrene-ethylene-propylene-styrenetriblock copolymer.
 20. The material of claim 1 wherein:said particulatefiller is a quantity of dielectric filler chosen to provide to saidelectrical substrate material a preselected dielectric constant, saidadded filler being dispersed substantially uniformly throughout saidcomposition.
 21. The material of claim 20 wherein:said dielectric filleris selected from the group consisting of titanium dioxide, bariumtitanate, strontium titanate, silica, fused amorphous silica, corundum,wollastonite, polytetrafluoroethylene, aramide fibers, fiberglass, Ba₂Ti₉ O₂₀, glass spheres, quartz, boron nitride, aluminum nitride, siliconcarbide, beryllia, alumina or magnesia.
 22. The material of claim 21wherein a plurality of said fillers having different dielectricconstants are employed, the respective quantities of said fillers beingselected to provide to said article a preselected dielectric constant.23. The material of claim 1 wherein said particulate filler is presentin an amount of between 30 and 50% volume percent.
 24. The material ofclaim 1 wherein said woven fabric is present in an amount of between 15and 25 volume percent.
 25. The material of claim 24 wherein said wovenfabric is present in an amount of between 15 and 25 volume percent. 26.The material of claim 1 further including:a metal layer having apreselected coefficient of thermal expansion and; said filler and thequantity thereof providing to said electrical substrate material acoefficient of thermal expansion substantially equal to that of saidmetal layer.
 27. The material of claim 1 further including:acrosslinking agent to said thermosetting composition.
 28. The materialof claim 27 wherein:said crosslinking agent is selected from the groupconsisting essentially of triallyl cyanurate, diallyl phthalate,divinylbenzene, a multifunctional acrylate monomer, or a combinationthereof.
 29. The material of claim 1 wherein said electrical circuitmaterial comprises a microwave circuit substrate.
 30. The material ofclaim 1 including:a flame retardant.
 31. The material of claim 30wherein:said flame retardant comprises a bromine containing flameretardant.
 32. The material of claim 1 wherein:said electrical circuitmaterial has a dissipation factor of equal to or less than 0.007. 33.The material of claim 1 wherein said unsaturated butadiene or isoprenecontaining polymer is selected from the group consisting of:(1) randomand block copolymers of primarily 1,3-addition butadiene or isoprenewith styrene, alpha-methyl styrene, acrylate or methacrylate, oracrylonitrile monomers, (2) copolymers of ethylene, (3) natural rubber,(4) norbornene polymers, (5) hydrogenated diene polymers and (6)butadiene-acrylonitrile copolymers.
 34. The material of claim 1 whereinsaid unsaturated butadiene or isoprene containing polymer compriseshomopolymers or copolymers of ethylene selected from the groupconsisting of:polyethylene, and ethylene-propylene-diene terpolymers.35. The material of claim 1 including:a free radical cure initiator. 36.An electrical circuit material consisting essentially of:(a) athermosetting composition comprising a polybutadiene or polyisopreneresin and an unsaturated butadiene or isoprene containing polymercapable of participating in crosslinking with said polybutadiene orpolyisoprene resin in an amount of from 25 to 50 volume percent withrespect to the entire electrical circuit material, said polybutadieneand polyisoprene resin being chemically distinct from said unsaturatedbutadiene or isoprene containing polymer; (b) a woven fabric in anamount of from 10 to 40 volume percent with respect to the entireelectrical circuit material; (c) a particulate filler in an amount offrom 5 to 60 volume percent with respect to the entire electricalcircuit material; (d) a free radical cure initiator; and (e) a flameretardant.
 37. The material of claim 36 wherein:said flame retardantcomprises a bromine containing flame retardant.
 38. An electricalcircuit material consisting essentially of:(a) an electrical substrate,said substrate including; a thermosetting composition comprising apolybutadiene or polyisoprene resin and an unsaturated butadiene orisoprene containing polymer capable of participating in crosslinkingwith said polybutadiene or polyisoprene resin in an amount of from 25 to50 volume percent with respect to the entire electrical circuitmaterial, said polybutadiene and polyisoprene resin being chemicallydistinct from said unsaturated butadiene or isoprene containing polymer;a woven fabric in an amount from 10 to 40 volume percent with respect tothe entire electrical circuit material; and a particulate filler in amamount of from 5 to 60 volume percent with respect to the entireelectrical circuit material; and (b) at least a portion of saidsubstrate including a layer of conductive material.
 39. The material ofclaim 38 wherein:said conductive material comprises metal laminated tosaid substrate.
 40. The material of claim 38 including:a free radicalcure initiator.
 41. The material of claim 38 including a flameretardant.
 42. The material of claim 40 including a flame retardant. 43.The material of claim 42 wherein:said conductive material comprisesmetal laminated to said substrate.